Occasionally, telephone calls are not successfully completed because the called station is either busy or does not answer. Less frequently, calls are not completed because all available communication paths through the switching network are busy. Upon encountering either type of busy condition, a caller usually reoriginates and redials the called number in a retry attempt to complete the call.
Call retry service has long been an available telephone service to eliminate the undesired calling party effort during the persistence of a busy condition. In some switching systems, the service is manually provided by an operator at the calling party expense and on a recall basis as disclosed, for example, in P. W. Wadsworth U.S. Pat. No. 3,248,486 of Apr. 26, 1966.
Significant advances have been made in recent years to the extent that switching systems are now capable of automatically retrying interoffice calls without operator assistance whenever they cannot initially be completed due to either busy or no answer conditions. One such system is described in U.S. Pat. No. 3,492,433, D. M. Duft et al. issued Jan. 27, 1970. The patent discloses program controlled electronic facilities in a Traffic Service Position System for automatically retrying calls without operator assistance and at a time specified by the calling party.
In the foregoing call retry arrangements, a retried call often has not been successfully completed when reattempted because, for example, the called station remained busy or the calling station became busy in the interim. To provide more assurance that such a call retry is successful when attempted, the art has progressed in recent years to the point where automatic callback equipment is now commercially available for retrying intraoffice calls in such a manner that the called station is checked to determine that it is idle before any reattempt is made to complete call connections.
While such equipment is a valuable adjunct to switching systems, its utility has proven to be limited to serving intraoffice calls and especially those served, for example, within a single private branch exchange. A persistent problem in the art has been that no facilities have heretofore been available for providing automatic callback service between one or more switching offices in such a way that interoffice call retry attempts are initiated only after it has been ascertained that the called station has become idle. Obviously, such a problem is particularly aggravated for long distance calls involving many switching centers and miles of transmission facilities. The problem has also proven undesirable from a telephone company standpoint because it consumes valuable switching time and energy resources and, in many cases, without any revenue for the noncompletable interoffice calls. In addition, the calling parties are inconvenienced by being reengaged in the futile retry processing of interoffice calls.